Metallic nanoparticles (MNPs) have generated extensive research interest during the past few decades due to their fascinating optical, electronic and catalytic properties. Since the properties of MNPs are size and shape dependent, significant research efforts have been devoted to the controlled synthesis of MNPs with anisotropic geometries, such as gold nanorods (AuNRs). Due to the geometrical change, AuNRs exhibit substantially different properties compared to spherical gold nanoparticles, one of which is their plasmonic properties. Unlike spherical gold nanoparticles, which give rise to one single surface plasmon resonance (SPR) band in the extinction spectrum, the SPR for AuNRs splits into two modes. The electron oscillation along the short axis and long axis of the nanorods results in the appearance of a transverse SPR band and a much stronger longitudinal SPR band, respectively. Besides the strong light adsorption, the longitudinal SPR band of the AuNRs can be effectively tuned from the visible to the near-infrared region, facilitating easy coupling to commercial laser sources. Thus, AuNRs have become very promising building blocks for surface enhanced Raman scattering (SERS) substrates.
One critical feature of an effective SERS substrate is the presence of ‘hot spots’ arising from the plasmon coupling between closely packed metallic nanostructures. Various techniques have been used to produce such highly dense nanorod assemblies, including solvent evaporation, Langmuir-Blodgett assembly, assembly driven by the interaction between the capping molecules on nanorod surfaces, assembly at a liquid-liquid interface, etc. Unfortunately, the applicability of these approaches has generally been limited to two-dimensional (2D) planar substrates, i.e., sheet-like substrates. For sensing applications, however, three-dimensional (3D) substrates with hierarchical structures (e.g., fibrous meshes, micro- or nanosphere aggregates etc.) are preferred due to the enhanced surface area, which leads to better detection. To produce MNP assemblies on 3D structures, substrates carrying metal-affinitive functional groups (e.g., thiol groups, pyridyl groups, etc.) have generally been used, and the exclusive chemistry occurring between the MNPs and the substrate provides the driving force for MNP immobilization. Since this type of MNP immobilization relies on the formation of specific chemical bonds, application of such methods is confined to a limited range of substrate materials. Thus, the ability to immobilize MNP on 3D substrates made from a variety of materials would be a welcome addition to the art.